Prior art blends of polyethylenes are discussed, for instance, in U.S. Pat. No. 4,339,507. One component of this blend is a linear low density copolymer of ethylene and octene and a second component is a low density polyethylene homopolymer. An example of such a linear low density polyethylene blend is Dow Chemical Company's Linear Low Density Polyethylene 3010. Such blends are typically useful as extrusion coatings onto various structures, such as flexible polymeric film/paper packages for foods, and metallized polymeric film balloons. The coatings serve as heat seal media and as barriers to protect the contents of a package from outside contamination, or to retain the contents, such as a gas such as helium, within a coated and sealed balloon. The ethylene-octene copolymer component of the prior art two-component blends is used to provide strong heat seal strengths required in certain extrusion coated structures.
However, these prior art blends create many problems in an extrusion coating operation. In particular, the blends require excessively high extruder amps to extrude; some extruders are not rated for these high amps. The high amps requirement is the reason that low density polyethylenes are combined with the copolymers, usually at about 20 weight percent of the low density polyethylene, to enhance processability. Also, because these prior art blends exhibit high viscosities at the high shear rates associated with extrusion coating, they tend to generate excessive melt temperatures in the feed sections of extruder screws. These high melt temperatures in the feed section cause extruder output variations, or surging, along with melt fracture and gel formation, all of which introduce quality defects. The prior art blends also exhibit high neck-in of the molten polymer web with its associated edge instability. The edge instability of the molten web requires that the edge be trimmed, which results in costly polymer waste. These facts are discussed in a technical article published in Converting Magazine, March 1997, entitled "Benefits, Processing Ease, Keep LDPE in the Mix".
The above technical article also reports that "performance capabilities of LDPE (low density polyethylene) are broadened by the addition of comonomers. Vinyl acetate and methyl acrylate are two primary examples." Ethylene vinyl acetate (EVA) copolymers permit lower heat seal initiation temperatures in those heat sealing operations requiring such low sealing temperatures. Ethylene methyl acrylate (EMA) copolymers display strong heat-seal characteristics for both film and extrusion coating applications."
Copolymers of ethylene and methyl acrylate, containing about 2weight percent to about 5 weight percent methyl acrylate, are thermally stable. However, the prior art blends containing linear low density copolymers of ethylene and octene (or any other .alpha.-olefin), and the low density copolymers of ethylene and vinyl acetate, containing about 2 weight percent to about 18 weight percent vinyl acetate, are thermally unstable. Either copolymer will degrade if it remains for too long in a heated extruder barrel, such as during a temporary operational shutdown. Such degradation, or molecular breakdown, generates massive defects in subsequent extrusion coatings. In most cases, the extrusion operator must clean his equipment before he can continue with production. Because of the copolymers' being prone to thermal degradation, these copolymers must be purged from the extruder before extruder screw rotation is stopped. The purge material is usually an inert polymer such as a low density polyethylene homopolymer. Such an inert polyethylene homopolymer can remain in an extruder until that extruder's next operational startup, and it can remain in an extruder through the heat-up cycle necessary to reach the extruder temperatures required to extrusion coat a copolymer. This need for purging the thermally unstable copolymers from extruders is materially costly, with the purge material having to be purchased and inventoried at the site of the extrusion coating operation.
Considering the problems associated with extrusion coating the above two copolymers, there is need for a more ideal homopolymer based on ethylene. Such a polymer would be an inert homopolymer of ethylene, extrusion coatings of which would be capable of equaling or exceeding the heat seal strengths of linear low density copolymers of ethylene and octene (or any other .alpha.-olefin), or the low density copolymer of ethylene and vinyl acetate. Such an ideal homopolymer would exhibit none of those problems of a linear low density polyethylene blend; that is, the ideal homopolymer would not require high extruder drive amps. Nor would it exhibit high viscosities due to high shear rates, which, in turn, generate excessive melt temperatures, melt fracture, and gels. Nor would such an ideal homopolymer generate high web neck-in, such high neck-in manifesting itself in unstable web edges, which must be trimmed away with loss of material. Finally, such an ideal homopolymer would not require purging each time an extrusion operation is interrupted, and it could remain in the extruder indefinitely between production start-ups.